全文获取类型
收费全文 | 1530篇 |
免费 | 37篇 |
国内免费 | 5篇 |
专业分类
化学 | 1144篇 |
晶体学 | 13篇 |
力学 | 19篇 |
数学 | 215篇 |
物理学 | 181篇 |
出版年
2022年 | 15篇 |
2021年 | 16篇 |
2020年 | 25篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 25篇 |
2014年 | 35篇 |
2013年 | 60篇 |
2012年 | 84篇 |
2011年 | 88篇 |
2010年 | 57篇 |
2009年 | 38篇 |
2008年 | 70篇 |
2007年 | 63篇 |
2006年 | 72篇 |
2005年 | 84篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 55篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 16篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 31篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 15篇 |
1975年 | 10篇 |
1974年 | 15篇 |
1973年 | 16篇 |
1970年 | 9篇 |
1882年 | 7篇 |
排序方式: 共有1572条查询结果,搜索用时 15 毫秒
961.
Gleb B. Sukhorukov Alexei A. Antipov Andreas Voigt Edwin Donath Helmuth Mhwald 《Macromolecular rapid communications》2001,22(1):44-46
pH‐Controlled encapsulation in and release of macromolecules from polyelectrolyte capsules of a few microns in diameter is demonstrated. Capsules were prepared via alternating adsorption of the oppositely charged polymers poly(allylamine hydrochloride) and poly(styrene sulfonate) onto decomposable melamin formaldehyde cores. The capsules were open for macromolecules at pH values below 6 and closed at pH > 8. 相似文献
962.
Edwin W. J. Hooijschuur Charles E. Kientz Udo A. Th. Brinkman 《Journal of separation science》2000,23(4):309-316
The potential of the on‐line coupling of microcolumn liquid chromatography (μLC) using aqueous eluents with a flame ionization detector (FID) was evaluated. An eluent‐jet interface was modified to allow the efficient introduction of the eluent into the FID. The potential of the method is demonstrated by the μLC–FID determination of lower alcohols and bis(2‐hydroxyethylthio)alkanes on porous and non‐porous stationary phases, respectively. Flow injection analysis (FIA)–FID experiments with highly polar, thermolabile, semi‐volatile and non‐volatile compounds like amino acids, organic acids, alkylphosphonic acids, and carbohydrates showed the developed configuration to be a promising approach for the detection of a wide range of analytes. Compared with a nebulization interface, the eluent‐jet interface showed 4–10 times higher peaks for citric acid. Detection limits by FIA for all compounds were in the range of 0.2–5 ng injected. With ribose as test compound, plots of peak height vs. amount injected showed good linearity (r2 > 0.999) in the range of 75–12,000 μg/mL. The repeatability showed relative standard deviations of less than 5%. 相似文献
963.
Mino R. Caira Luigi R. Nassimbeni Dejana Vujovic Edwin Weber Andreas Wierig 《Structural chemistry》1999,10(3):205-211
Structures of the inclusion compounds formed between the host compound 1,4-bis(9-hydroxy-9-fluorenyl)benzene and three lutidine regioisomers, 2,4-, 3,5-, and 2,6-lutidine, have been elucidated. Competition experiments were performed to investigate the degree of host selectivity for complexation with the lutidine isomers. Lattice energy calculations explain the results of the competition experiments, and thermal analysis shows that the desolvation reactions are complex. 相似文献
964.
The ship capsizing problem is one of the major challenges in naval architecture. The IMO criterion regarding capsize stability is still the righting lever curve of static stability calculated for calm water. For the prediction of large–amplitude motions the dynamic loads have to be included. The capsizing of a ship in regular waves is resulting from a sequence of bifurcations in the ship's motion: The determination of bifurcations is possible using path‐following techniques of nonlinear dynamics. Existing tools are, however, without adaption not readily applicable for the determination of bifurcations. The main and until now unsolved problem is the necessity of including memory integrals to describe the ship hydrodynamics. First results with simple algorithms show two different scenarios leading to capsizing due to increasing wave amplitudes. 相似文献
965.
966.
Ivan Lopez-Arevalo Edwin Aldana-Bobadilla Alejandro Molina-Villegas Hiram Galeana-Zapin Victor Muiz-Sanchez Saul Gausin-Valle 《Entropy (Basel, Switzerland)》2020,22(12)
The most common machine-learning methods solve supervised and unsupervised problems based on datasets where the problem’s features belong to a numerical space. However, many problems often include data where numerical and categorical data coexist, which represents a challenge to manage them. To transform categorical data into a numeric form, preprocessing tasks are compulsory. Methods such as one-hot and feature-hashing have been the most widely used encoding approaches at the expense of a significant increase in the dimensionality of the dataset. This effect introduces unexpected challenges to deal with the overabundance of variables and/or noisy data. In this regard, in this paper we propose a novel encoding approach that maps mixed-type data into an information space using Shannon’s Theory to model the amount of information contained in the original data. We evaluated our proposal with ten mixed-type datasets from the UCI repository and two datasets representing real-world problems obtaining promising results. For demonstrating the performance of our proposal, this was applied for preparing these datasets for classification, regression, and clustering tasks. We demonstrate that our encoding proposal is remarkably superior to one-hot and feature-hashing encoding in terms of memory efficiency. Our proposal can preserve the information conveyed by the original data. 相似文献
967.
Johnny Aldo Tinco-Jayo Enrique Javier Aguilar-Felices Edwin Carlos Enciso-Roca Jorge Luis Arroyo-Acevedo Oscar Herrera-Calderon 《Molecules (Basel, Switzerland)》2022,27(1)
Jatropha macrantha Müll Arg. L is also known as “huanarpo macho” and used in the Peruvian traditional medicine as an aphrodisiac and erectile dysfunction (ED). The aim of this study was to determine the phytochemical constituents in leaves and stems ethyl acetate fraction (LEAF and SEAF) of J. macrantha and to compare the antioxidant activity and the ameliorative effect on ketamine-induced erectile dysfunction in rats. The phytochemical constituents were determined by LC-ESI-MS/MS, the total phenolic compounds and total flavonoids (TPC and TF) by Folin-Ciocalteu and aluminum chloride, respectively. The antioxidant activity was determined by DPPH, ABTS, and FRAP assays. Experimental groups were divided as follows: I: negative control; II: positive control (ketamine at 50 mg/ kg/d); III: sildenafil 5 mg/kg; IV, V, VI: LEAF at 25, 50 and 100 mg/kg, respectively, and VII, VIII, IX: SEAF at 25, 50, and 100 mg/kg, respectively. The phytochemical analysis revealed the presence mainly of coumarins, flavonoids, phenolic acids, and terpenes. TPC of LEAF and SEAF were 359 ± 5.21 mg GAE/g and 306 ± 1.93 mg GAE/g, respectively; TF in LEAF and SEAF were 23.7 ± 0.80 mg EQ/g, and 101 ± 1.42 mg EQ/g, respectively. The DPPH, ABTS, FRAP in SEAF were 647 ± 3.27; 668 ± 2.30; and 575 ± 2.86 μmol TE/g, respectively, whilst LEAF showed 796 ± 3.15; 679 ± 0.85; and 806 ± 3.42 μmol TE/g, respectively. Regarding sexual behavior, LEAF showed a better effect in mount frequency, intromission frequency, ejaculation frequency, mount latency, intromission latency, ejaculatory latency, and post ejaculatory latency than SEAF. As conclusion, LEAF of J. macrantha at 50 mg/kg showed a better effect on sexual behavior in male rats with erectile dysfunction than SEAF but not higher than sildenafil. 相似文献
968.
A random synthetic jet array driven turbulence tank 总被引:1,自引:0,他引:1
Evan?A.?VarianoEmail author Eberhard?Bodenschatz Edwin?A.?Cowen 《Experiments in fluids》2004,37(4):613-615
We measure the flow above an array of randomly driven, upward-facing synthetic jets used to generate turbulence beneath a free surface. Compared to grid stirred tanks (GSTs), this system offers smaller mean flows at equivalent turbulent Reynolds numbers with fewer moving parts.
相似文献
Evan A. VarianoEmail: |
969.
970.
Edwin Weber J Oouml;rg Trepte Victor Ch. Kravtsov Yurii A. Simonov Edward V. Ganin Janusz Lipkowski 《Journal of inclusion phenomena and macrocyclic chemistry》2000,36(3):247-257
The X-ray crystal structure of the solvent inclusion compound formed between heterocalix[8]arene 1, ethyl acetate and water (1 : 1 : 4.5) has been studied. The compound crystallized in tetragonal space group I41/a, a = b = 21.278(3), c = 31.290(4) Å, V = 14167(4) Å3, Z = 8. The host molecule, incorporating benzimidazol-2-one and 2,5-dimethoxy-1,3-phenylene subunits in an alternate cyclic arrangement, forms an almost perfectly closed cavity which encapsulates one solvent ethyl acetate guest molecule. Water molecules being entrapped in the lattice cages in the form of cyclic and linear clusters bind the bulky inclusion complexes via H-bonds in infinite layers. Two symmetry center related benzimidazole-2-one moieties of two hosts from neighbouring layers in the crystal lattice are arranged such that they partially overlap exhibiting stacking interaction. 相似文献